Question: Kevin is 24 years older than Stephanie. Six years ago, Kevin was 4 times as old as Stephanie. How old is Stephanie now?
Solution: We can use the given information to write down two equations that describe the ages of Kevin and Stephanie. Let Kevin's current age be $k$ and Stephanie's current age be $s$ The information in the first sentence can be expressed in the following equation: $k = s + 24$ Six years ago, Kevin was $k - 6$ years old, and Stephanie was $s - 6$ years old. The information in the second sentence can be expressed in the following equation: $k - 6 = 4(s - 6)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $s$ , it might be easiest to use our first equation for $k$ and substitute it into our second equation. Our first equation is: $k = s + 24$ . Substituting this into our second equation, we get the equation: $(s + 24)$ $-$ $6 = 4(s - 6)$ which combines the information about $s$ from both of our original equations. Simplifying both sides of this equation, we get: $s + 18 = 4 s - 24$ Solving for $s$ , we get: $3 s = 42$ $s = 14$.